(a) Field of the Invention
The present invention is related to a light emitting diode and a fabrication method thereof, and in particular, to a light emitting diode fabricated by growing single crystal thin films through controlling the crystal orientation of the substrate.
(b) Description of the Related Art
A light emitting diode is a photoelectric conversion element having a structure such that an n-type semiconductor which has electrons as a carrier and a p-type semiconductor which has holes as a carrier are welded to each other.
In order to convert electrical energy into light, a voltage is set up on both electrode terminals of the light emitting diode, and a current is applied in one direction. The electrons and the holes recombine and a part of the energy generated by the recombination is converted into light.
The light emitting diode is used for a display when the wavelength of the light emitted from the light emitting diode is in the visible region, it is used for various excitation light sources when the wavelength is in the ultraviolet region, and it is used as an element that transfers information between machines when the wavelength is in the infrared region.
The performance of a light emitting diode that emits visible light is determined by light emitting efficiency (luminous efficiency), and the light emitting efficiency is indicated as lumens per watt.
The light emitting efficiency of the light emitting diode is mainly dependent on the three factors of internal quantum efficiency, extraction efficiency, and operating voltage.
The internal quantum efficiency is a characteristic value of the amount of photons for the amount of electrons passing through the light emitting diode, and it is determined by the quality of the semiconductor material and the design of the active portion.
The extraction efficiency is a ratio of the amount of photons extracted from the semiconductor chip to the amount of photons generated in this way.
Many deflections of the photons occur due to a large difference of the refractive index between the semiconductor and the other material, and some of the photons are absorbed by the semiconductor chip during the deflection. Accordingly, the extraction efficiency is limited by the deflection of the photons in the semiconductor chip.
The external quantum efficiency is a product of the internal quantum efficiency and the extraction efficiency.
Recently, InGaAlP and InGaN light emitting diodes that were grown with metal organic chemical vapor deposition have been developed. InGaAlP is a III-V group compound semiconductor of direct transition that can be matched to a GaAs substrate and that has high internal quantum efficiency.
In spite of the development of technologies, the efficiency of extraction of photons that can be extracted from the inside to the outside of the light emitting diode, that is, the extraction efficiency, has a fundamental limitation due to the internal loss and the large refractive index of the light emitting diode.
Methods for improving light extraction efficiency are under constant development. As one of these efforts, a method of forming concavo-convexes on the substrate is known.
The concavo-convexes are formed on the substrate through a photolithography and dry etching technique, and single crystal thin films are grown on the substrate which the concavo-convexes are formed on.
A diffused reflection of light emitted from the active layer occurs due to the concavo-convexes formed on the interfaces of the single crystal thin films so that the light emitting efficiency is improved.
However, the process of forming the concavo-convexes on the substrate is very complex because of the photolithography and dry etching technique. In addition, it is difficult to grow the single crystal thin films on a plane that has the concavo-convexes, and the cost of fabricating the light emitting diode is too high.